The unborn baby spends around 37 weeks in the uterus (womb), but the average length of pregnancy, or gestation, is calculated as 40 weeks. This is because pregnancy is counted from the first day of the woman’s last period, not the date of conception which generally occurs two weeks later, followed by five to seven days before it settles in the uterus. Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a pregnancy is considered full term if birth falls between 37 to 42 weeks of the estimated due date.
A baby born prior to week 37 is considered premature, while a baby that still hasn’t been born by week 42 is said to be overdue. In many cases, labour will be induced in the case of an overdue baby.
Length of gestation
The average length of human gestation is 280 days, or 40 weeks, from the first day of the woman’s last menstrual period. The medical term for the due date is estimated date of confinement (EDC). However, only about four per cent of women actually give birth on their EDC. There are many online pregnancy calculators (see Baby due date calculator
that can tell you when your baby is due, if you type in the date of the first day of your last period.
A simple method to calculate the due date is to add seven days to the date of the first day of your last period, then add nine months. For example, if the first day of your last period was 1 February, add seven days (8 February) then add nine months, for a due date of 8 November.
Determining baby due date
Irregular menstrual cycles can mean that some women aren’t sure of when they conceived. Some clues to the length of gestation include:
- Ultrasound examination (especially when performed between six and 12 weeks)
- Size of uterus on vaginal or abdominal examination
- The time fetal movements are first felt (an approximate guide only).
A pregnancy ultrasound is a non-invasive test that scans the unborn baby and the mother’s reproductive organs using high frequency sound waves. The general procedure for a pregnancy ultrasound includes:
- The woman lies on a table.
- A small amount of a clear, conductive jelly is smeared on the woman’s abdomen.
- The operator places the small hand-held instrument called a transducer onto the woman’s abdomen.
- The transducer is moved across the abdomen. The sound waves bounce off internal structures (including the baby) and are transmitted back to the transducer. The sound waves are then translated into a two-dimensional picture on a monitor. The mother doesn’t feel or hear the transmission of the sound waves.
- By measuring the baby’s body parts, such as head circumference and the length of long bones, the operator can estimate its gestational age.
The diagnostic uses of pregnancy ultrasound
Apart from helping to pinpoint the unborn baby’s due date, pregnancy ultrasounds are used to diagnose a number of conditions including:
- Multiple fetuses
- Health problems with the baby
- Ectopic pregnancy (the embryo lodges in the fallopian tube instead of the uterus)
- Abnormalities of the placenta such as placenta praevia, where the placenta is positioned over the neck of the womb (cervix)
- The health of the mother’s reproductive organs.
A baby born prior to week 37 is considered premature. The odds of survival depend on the baby’s degree of prematurity. The closer to term (estimated date of confinement, or EDC) the baby is born, the higher its chances of survival - after 34 weeks gestation with good paediatric care almost all babies will survive.
Premature babies are often afflicted by various health problems, caused by immature internal organs. Respiratory difficulties and an increased susceptibility to infection are common.
Often there is no known cause for a premature labour; however, some of the maternal risk factors may include:
- Drinking alcohol or smoking during pregnancy
- Low body weight prior to pregnancy
- Inadequate weight gain during pregnancy
- No prenatal care
- Emotional stress
- Placenta problems such as placenta praevia
- Various diseases such as diabetes and congestive heart failure
- Infections such as syphilis.
Around five out of every 100 babies will be overdue, or more than 42 weeks gestation. If you have gone one week past your due date without any signs of impending labour, your doctor will want to closely monitor your condition. Tests include:
- Monitoring the fetal heart rate
- Using a cardiotocograph machine
- Performing ultrasound scans.
The placenta starts to deteriorate after 38 weeks or so, which means an overdue baby may not get enough oxygen. An overdue baby could also grow too large for vaginal delivery. Generally, an overdue baby will be induced once it is two weeks past its expected date. Some of the methods of induction include:
- Vaginal prostaglandin gel - to help dilate the cervix
- Amniotomy - breaking the waters, sometimes called an artificial rupture of membranes (ARM)
- Oxytocin - a synthetic form of this hormone is given intravenously to stimulate uterine contractions.
Where to get help
- Your doctor
- Your obstetrician
- Midwife or childbirth educator
Things to remember
- The unborn baby spends around 38 weeks in the uterus, but the average length of pregnancy, or gestation, is counted at 40 weeks.
- Pregnancy is counted from the first day of the woman’s last period, not the date of conception which generally occurs two weeks later.
- Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a baby is considered full term if its birth falls between 37 to 42 weeks of its estimated due date.
This page has been produced in consultation with and approved by:
Royal Australian and New Zealand College of Obstetricians and Gynaecologists
Page content currently being reviewed.
Content on this website is provided for education and information purposes only. Information about a therapy, service, product or treatment does not imply endorsement and is not intended to replace advice from your doctor or other registered health professional. Content has been prepared for Victorian residents and wider Australian audiences, and was accurate at the time of publication. Readers should note that, over time, currency and completeness of the information may change. All users are urged to always seek advice from a registered health care professional for diagnosis and answers to their medical questions.